Concentrating solar power systems are designed to collect heat by absorbing sunlight. Classically, sunlight is concentrated and focused onto a solar absorber body containing a heat transfer fluid (steam, oil or other heat transfer medium). The solar absorber body can be, for example an elongated absorber tube through the interior of which flows the heat transfer fluid. The solar absorber body absorbs the sunlight and convert it into heat. Then heat is transmitted to the heat transfer fluid. The temperature of the fluid then strongly increases.
The heat transfer fluid can be used in a standard turbine generator for electricity generation. For example, parabolic troughs, Fresnel reflectors and solar power towers can be used to convert the sunlight into thermal electric power. Generally, the solar absorber body contains a substrate, usually an elongated tube, covered by a selective coating comprising an infrared reflective layer, a solar absorption layer and most often an antireflective layer.
The selective coating must present specific optical properties such as a high solar absorption and a low thermal emissivity. Moreover, the selective coating must be thermally stable without impairing the reflective and absorbing properties. The lifetime of the selective coating depends, among others, on the working atmosphere, the maximal operating temperature and on the variation of temperatures at the outer surface of the tube.
The tube of the solar absorber body is subjected to irradiation only on a semi-cylindrical surface, i.e. where the sunlight is focused. Therefore, the tube is subjected to large circumferential heat flux variations on its outer surface inducing circumferential thermal gradient on the outer surface of the tube. These thermal gradients induce on one hand thermo-mechanical stresses and on the other hand area of higher temperature, which accelerate the deterioration of the selective coating.
As represented in FIG. 1, to reduce the circumferentially thermal gradient, international application WO2011/055401 proposes to introduce protruding elements on the irradiated portion of the tube, on the inner surface of the tube, to increase the turbulence of the fluid and therefore the exchanged thermal energy. These elements can be fins, all with radial extension on the side of the tube where the sunlight is focused.
The presence of fins in a portion of the solar tube allows to locally lower the thermal circumferential gradients. A parametric analysis has been conducted as a function of the fins height, the number of fins, the angle disposition of the fins in the tube and the temperature of the heat transfer medium for a level of convective heat transfer coefficient of 520 W/m2·K, which is close to those obtained for thermodynamic solar power applications using gas as heat-carrying fluid. For instance, the maximum temperature passes from 370° C. in absence of fins down to 323° C. using 24 fins 5 mm long and 2 mm thick distributed over an arc of 150° in the inner surface of the tube. The presence of fins reduces deformation of the section of the tube by 10%.
However, the section of the obtained tube is not axisymmetric and therefore requires particular and complicated processes of elaboration.
International application WO2012/110341 describes a solar tube, the inner face of the tube being provided with helical ribs. Nevertheless, the thermal circumferential gradient reduction with such a structuration would be negligible for solar applications involving heat transfer temperature reaching 500° C.